How to parent eye armature to head bone in body armature
Your question sounds like you have two different armatures (which are objects), you're selecting both and then entering edit mode (which in this case will be a "double" edit mode, working simultaneusly on two different objects). But the parenting you're looking for is possible only internally to one single object. 

So, select the eye armature, shift select the head armature, press Ctrl J (join two objects into a single one).

Then select the new object (armature), enter edit mode, select the eye bone, shift select the head bone and press Ctr P (keep offset).

And, yes, teeth and head can be treated in the same way (create the new bones in edit mode with shift A or Shift D).